In the specialised nursing field of acute mental illness nurses expressed a need to measure and evaluate their patients' mental-health outcomes both empirically and routinely. The aim was to develop and test a measurement tool, named the DELTA nursing measure, which could be embedded routinely into the nursing process and care plans, enabling the psychiatric nurses to score and evaluate their patients' acute mental-health outcomes. A qualitative, exploratory study design was used to address two sequential objectives. Firstly, qualitative data that described observable behaviours in patients with acute mental illness were collected from psychiatric nurses (n = 5) who were experienced in acute mental healthcare. The data were analysed using inductive content analysis techniques to design and construct the DELTA nursing measure. In the second objective, the nursing utility of the DELTA nursing measure was studied. This was done by training and testing a new team of psychiatric nurses (n = 25) working in a 116-bed acute psychiatric hospital, in the application of the DELTA nursing measure. After 30 months a focus group (n = 6) representing this team was held to explore their perceptions and experiences of the nursing utility of the newly-developed measure. The descriptive data were analysed using deductive content analysis techniques. The outcome of the DELTA nursing measure as a routine nursing measure of acute mental illness provided good results. The nursing-utility characteristics have confirmed positive responses with regard to its acceptance, usefulness and confidence as a worthwhile tool to be used in expediting nursing services in acute mental healthcare. The positive responses to the DELTA nursing measure are noteworthy. It has the potential to add substantial value to the mental health care field in nursing by adding a measurable dimension to patient outcomes, a much needed requirement by patients, multidisciplinary teams and healthcare funders.